This invention relates to industrial apparatus, namely a mill, in which pieces of a material are pulverised into a finer particulate form. The invention relates particularly, but not exclusively, to a mill in which coal is pulverised into a powder form which is conveyed to combustion apparatus e.g. of a power station.
In particular the invention concerns a mill having a lower grinding member, which may be a grinding ring formed with an annular depression. Grinding elements are sandwiched between the lower grinding member and a top member, which may be a top ring.
Typically the required relative movement between the grinding elements and the lower grinding member is achieved by driving the grinding member, while the top member is held against rotation. The grinding elements, which are typically steel balls or rollers, are not driven. They may be fixed or free to precess.
The mill with which the invention is concerned is of the type having a rotating port ring generally as described in EP 0507983. Such a port ring is provided, between the periphery or circumference of the grinding ring and the wall of the mill. There is provided an annular passage or “throat”, just outboard of the grinding ring. Air flows upwardly through the port ring. The port ring has a plurality of spaced-apart vane members. The port ring rotates with the grinding ring and the vane members to impart a desired vector to the generally upwards air flow.
The rotating port ring is an excellent and successful mill feature but we have realised that the provision of a fixed area for the passage of upwardly impelled air is a limitation. Coal sources vary, as do desired operating speeds and coal throughputs. Increasing the airflow rate e.g. to accommodate an increase in coal throughput means increasing the air speed, and that increases the risk that non-combustible mineral materials may be carried forward to the combustion apparatus, along with desired coal fines.